1. Field of the Invention
The present invention relates to AM-FM receivers adapted for integrated circuit fabrication and having improved mode conversion means.
The invention treats the means for converting the radio receiver from AM to FM and optionally to phono operation while at the same time optimizing the individual circuits for each mode of operation. Optimization is achieved in noise filtering, particularly in the FM mode, automatic gain control in AM, automatic frequency control in FM and temperature compensation in FM.
2. Description of the Prior Art
Radio receivers for AM and FM operation have been proposed fabricated primarily of integrated circuitry. In integrated circuit fabrication, increased circuit complexity produces little increase in costs, thereby permitting highly sophisticated circuitry even in low cost applications. Because of the cost advantage in integration, it is desirable to integrate a maximum amount of the circuitry with a minimum of non-integrated or "outboarded" components. Cost considerations also dictate a minimum pin count. In carrying out radio receiver functions, optimization of each function for either the AM or FM mode is desirable and the conversion between modes should be readily achieved. For instance, it is essential to provide an IF amplifier which is of a high and stable gain for FM operation, while for AM operation less gain is required and the gain should be readily controlled. Similarly, the detection function should be separately optimized for the AM and for the FM detection modes and for easy conversion between these two modes. A suitable IF amplifier is a d.c. coupled broad band amplifier incorporating a plurality of emitter coupled transistor stages and utilizing degenerative feedback for balance. Such an amplifier is disclosed in the first cited U.S. application. A suitable AM-FM detector is described in the second cited patent application. A known technique for enabling and disabling functional circuits of this nature is through the use of controlled transistor current sources which are in turn responsive to the setting of a manual switch.